1. Field of the Invention
The present invention relates to an image search technique based on the degree of similarity between feature points in images.
2. Description of the Related Art
In general, the image search corresponding to the rotation of an image is performed by obtaining a feature amount after the rotation of a query image, or by using a feature amount after rotational transform.
Japanese Patent Laid-Open No. 2006-065399 is an example of a conventionally proposed image search technique based on the degree of similarity between feature points in images. According to the document, feature points similar to a pair of feature points randomly selected from a query image are obtained from a registered image in the image search based on the comparison of local features. A geometric transformation function between the query image and the registered image is then obtained based on the positional relationship between the pair of feature points. Corresponding feature points in the registered image existing at places after the transformation of feature points other than the pair based on the geometric transformation function are counted to determine the similarity.
However, in the process, an image including a symmetric (horizontally/vertically symmetric) partial image may not be appropriately searched in the image search based on the comparison between the local features. More specifically, in a symmetric partial image, an incorrect combination of similar feature points may be extracted (a pair of similar feature points crosses symmetric areas), and the search may fail, as described later with reference to FIGS. 7A to 7C.